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The landing gear is one of those things that pretty much has to work. It has to be strong enough to support the weight of a fully loaded aircraft and handle the impact of my landings - note to self, buy concrete company stock.

For all that, it's one of those systems we don't give a lot of thought to, until something goes wrong with it.

Any aircraft design is a series of compromises. You want the landing gear to be strong enough but you don't want to overbuild it because it's just dead weight to carry around 90% of the time.

The landing gear on the Airbus 300 was a good example of "over built". You couldn't hurt it. Land hard, no problem. Land in a crab, sure.

On the other hand, the DC-10/MD-11 has a nasty habit of folding its main gear.

So the gear obviously has to support the weight of the aircraft. It also has to spread that weight across a large enough area to not overstress the pavement. That's why as aircraft get larger and heavier we see extra wheels being added. Note the six wheels on this 777 main gear:

Boeing 777 Main Gear
This Airbus 340 has an extra set of wheels in the center. MD-11s use a similar setup:
Airbus 340 with center gear
The most common setup is what's called tricycle gear - two main gear plus a nose gear. Smaller airliners like the Airbus 319 often get by with just two wheels on each main. Just about all medium sized airliners have 4 tires on each main. When you start getting into the big stuff you see crazy setups like this 747:
747 main gear (16 wheels)
or this A380 with 20 (count 'em) main wheels, a pair of sixes and a pair of fours.
A380 main gear (20 wheels)
Military transports which are designed to land on unprepared surfaces tend to have some really strange looking gear. They tend to have lots of wheels spread out over a large area to spread the weight out. Check out the wheels on this Antonov 124
Antonov 124 main gear
Just to give you an idea of how much weight we're supporting, this is what happens when an MD-11 gets about 5 feet left of where it's supposed to be:
MD-11 left main on unstressed pavement
In addition to supporting the plane on the ground the nose gear pivots to steer the aircraft. On most jets the rudder pedals give you a limited amount of nose gear steering, enough to keep you going straight down the runway. A tiller, usually on the Captain's left side, gives full control over the nose wheel.

You can make a very tight turn but it's rough on the main gear. If you use differential braking (hold the brake on the inside wheels) it will pretty much pivot around the inside main gear. Not something you want to do normally. At the very least you're going to scrub tires.

757 turn chart
Now, 99% of everybody knows this already, but I have been asked this: the main wheels are not driven by anything. The thrust from the engines is what makes the plane go on the ground.

Once airborne we'd like to retract the gear and get rid of all that drag. A lot of things have to happen all in the correct sequence for this to happen. A weight on wheels switch should prevent us from raising the gear on the ground. Just to be safe we wait until we get a positive climb indication "positive rate" before calling for gear up.

1. The gear doors need to open. They're normally closed even when the gear is down because you wouldn't want the extra drag on takeoff.

2. The main trucks need to level (on some aircraft). A leveling cylinder takes care of this.

3. The wheels need to stop spinning. On the B-52 we had to actually tap the brakes. Most planes built after the Jurassic era do this for you. Since the nose wheels normally don't have brakes a set of "snubbers" stop them from spinning.

4. The gear retracts. On most jets the mains retract inwards into the center of the fuselage because that's where there's enough room for them.

5. The up-locks engage. The gear will now stay put even if hydraulic pressure is removed.

6. The gear doors close.

7. Hydraulic pressure to the landing gear is switched off (most jets). Why? Because early jets (KC-135) didn't do this and when they got a hydraulic leak it was usually in the landing gear. The up-locks will hold it just fine, no sense risking a leak.

This is what happens if you get a little too eager to raise the gear:

MiG pilot raises gear prematurely

757 Gear Handle
Note the Off position in the middle
Extending the gear works pretty much the same in reverse:

1. The gear doors open.

2. The gear extends.

3. Once the main strut goes over center the gear is locked down. Even if we lose hydraulics it will stay put.

4. The gear doors close.

A310 Main Gear
Note how the main strut bends like a knee
Now suppose idiot-boy here forgets to put his landing gear down. The plane will sound a warning. The logic is different for every aircraft. This is what a 757 uses to determine something is amiss:

• The airplane is in flight, and
• Any landing gear is not down and locked, and
• Either of the following conditions exists:
- flaps in a landing position (25 or 30), or
- any thrust lever is at idle with radio altitude below 800 feet

In addition to all that, the Ground Proximity Warning System (GPWS) will also note that we're getting low without the gear extended and will start yelling "Too low! Gear!" at us.

And even with all that people have still accidentally landed gear up. Doh!

This is the indication we get in the cockpit. The three green lights mean all three gear are down and locked. If the yellow "Gear" light is lit that means the gear position "disagrees" with the gear handle. In simple terms handle is up and the gear (one or more) isn't or the handle is down and the gear isn't. If the yellow "Doors" light is lit that means that one or more of the gear doors is open.

757 Gear Indication
By now you're probably wondering "what if the gear doesn't come down?"

We've got that covered. Depending on the aircraft, the gear is designed to simply fall into place once the up-locks are released.

On the 727 this was a lot of work for the flight engineer. You would have to get the crank out, open three access panels on the cockpit floor, and for each wheel make 7 turns in one direction (to release the gear door), 7 turns in the opposite direction (to release the gear up-lock) and then 7 turns back in the original direction to lock the gear in place. Whew!

On the A300 I could, without getting out of my seat, insert a crank into the floor (it was easy to turn but took a lot of turns) and unlock all the gear. Definitely an improvement.

Ah, but on the 757 all I have to do is flip a switch, the up-locks are released electrically, and it all (literally) falls into place. Thanks Boeing engineers!

OK, suppose it just isn't our day and despite our best efforts some or all of the gear just won't budge.

If that's the case we're going to land on whatever we've got. Some gear is better than none. It will absorb some of the shock of landing and then we can (gently) let the plane settle once we get it slowed down a bit. Ideally we're going to burn off most of our gas and find a nice long runway.

Even a belly landing is not as hairy as you might think. Not that I ever want to do one, but normally the damage to the plane is pretty minimal. Here's a video of a Polish LOT 767 making a flawless gear up landing.

767 Gear up landing

That's about it. I was going to talk about the brakes, but I think stopping the airplane will require a whole chapter to do it justice. I'll leave you with this video that's been around for a few years but is still funny (and an amazing bit of CGI)


Originally posted to Major Kong on Fri May 17, 2013 at 08:02 AM PDT.

Also republished by Kossack Air Force, Aviation & Pilots, and Central Ohio Kossacks.

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